The present invention relates to a development apparatus in image forming apparatuses of an electrophotographic system, including copiers, printers, facsimile machines, and multifunction products thereof.
In a processing apparatus, such as a development apparatus, included in an image forming apparatus of an electrophotographic system, particularly with an increase in full-color output, there are demands for an extension of life, an increase in reliability, and an improvement in image quality in addition to miniaturization of the apparatus and an increase in output.
In the development apparatus, a so-called two-component development system in which developer having toner and carrier mixed therein is used is the mainstream. In the two-component development system, by continuously replenishing toner, images can be outputted for a long period of time. However, in a conventional two-component development system in which an amount of toner corresponding to an amount of toner consumed by development is replenished without discharging any developer from a development apparatus, degradation of carrier that develops with time is problematic.
Hence, in recent years, in terms of mainly an extension of life and an increase in reliability, a so-called trickle system has attracted attention in which by replenishing not only toner but also carrier in a development apparatus at an appropriate probability and discharging unnecessary carrier, carrier in the development apparatus is changed at a certain probability, whereby degradation of carrier is suppressed. In the trickle system, by adding carrier at a certain ratio to toner to be replenished and replenishing the carrier and the toner all together, the amount of developer in a developer apparatus is maintained at a certain level. Since a portion of developer that exceeds the certain level is discharged outside the development apparatus by a developer discharging mechanism, developer in the development apparatus is changed at a certain probability within a certain period of time. New carrier is continuously replenished, which in turn suppresses degradation of developer.
However, the so-called trickle system has another problem that does not arise in the conventional two-component development system. The problem will be described in detail below. In the conventional two-component development system, since the replenishing and discharging of carrier are not performed, the amount of developer in a development apparatus is always constant. For example, even when tilting of a development apparatus, variation in the fluidity of developer itself, or variation in the operating speed of a development apparatus occurs, since a development apparatus of the conventional two-component development system is, so to speak, a closed system, the amount of developer is maintained constant. However, in the trickle system, since, as described above, the refilling and discharging of carrier are performed, an increase or decrease in the amount of developer in a development apparatus may occur depending on the condition. In a developer discharging mechanism, a discharge force and a blocking force that restricts discharge act on developer. When tilting of the development apparatus, variation in the fluidity of developer itself, variation in the operating speed of the development apparatus, or the like, occurs, a discharge force and a blocking force against developer in the developer discharging mechanism vary. Due to the variation, an increase or decrease in the amount of developer in the development apparatus occurs. This indicates that developer circulation in the development apparatus that is balanced in the conventional two-component development system as a closed system can be disturbed in the trickle system by the influence of the developer discharging mechanism.
Particularly near the developer discharging mechanism, the influence of a discharge force that attempts to discharge developer and a blocking force that attempts to restrict the discharge force is remarkable. Accordingly, the circulation performance of developer near the developer discharging mechanism needs to be set taking into account these discharge force and blocking force, relative to circulation performance on the side where the developer discharging mechanism is not present. Specifically, near the developer discharging mechanism, due to the influence of these discharge force and blocking force, circulation performance is weakened as compared with that on the other side of the developer discharging mechanism. Accordingly, circulation performance sensitively reacts to the aforementioned tilting, variation in fluidity, variation in speed, or the like.
Hence, when the influence of a discharge force and a blocking force in the developer discharging mechanism is not taken into consideration, if the development apparatus is tilted, then depending on the up/down position of the developer discharging mechanism, discharging of developer may be promoted more or may be inhibited more over the case in which the development apparatus is in a normal position, which causes variation in the amount of developer in the development apparatus. Also, a change in circulation performance brought about by a change in the speed of the development apparatus leads to a change in the developer discharge performance of the developer discharging mechanism, which similarly causes variation in the amount of developer in the development apparatus.
A decrease in the amount of developer in the development apparatus leads to a decrease in the amount of developer supplied to a developer carrying member, which in turn causes problems such as image loss and a developer supply failure. On the other hand, an increase in the amount of developer in the developer apparatus causes stagnation of circulation of the developer in the development apparatus, which in turn causes problems such as insufficient stirring of developer and a developer supply failure.
Japanese Patent Application L,aid-open Publication No. 2006-323238 discloses a configuration in which, in a circulation transport system using spiral blades, the blades are partially eliminated near a developer discharging mechanism, a configuration in which the blades are allowed to partially have a double-threaded structure near the developer discharging mechanism, and a configuration in which a spiral blade oriented in an opposite direction is partially provided near the developer discharging mechanism. These configurations are intended to make the amount of developer in a development apparatus constant by suppressing the accumulation height of developer near the developer discharging mechanism and thereby making the amount of developer introduced into the developer discharging mechanism constant.
The configurations disclosed in Japanese Patent Application Laid-open Publication No. 2006-323238, however, have the following problems.
In the configuration in which the blades are partially eliminated and the configuration in which the blades are allowed to partially have a double-threaded structure, since a developer transport force near the developer discharging mechanism is reduced, developer present in this portion does not move or becomes difficult to move and as a result stays in the development apparatus for a long period of time. This is a phenomenon conflicting with the original purpose of the trickle system, that is, the prevention of degradation of carrier by changing developer.
In the configuration in which a blade oriented in an opposite direction is partially provided, since a developer transport force locally increases and thus a difference in transport speed occurs between the downstream side and the upstream side, retention of developer occurs and accordingly the amount of developer in the development apparatus is not stabilized.
Furthermore, when the rotational speed of the spiral blades is changed, a difference in energy between before and after the change is proportional to the square of the speed. Therefore, when the configurations of the spiral blades are partially changed, as in the configurations disclosed in Patent Document 1, variation in the amount of developer in the development apparatus caused by the change in rotational speed increases.